What you are looking for is a concentrated degreaser. I use the ZEP brand from Menards . it was made by Rubbermaid for a while but they changed brands . It is purple in color and comes in a blue one gallon jug. Just find a resealable container that you can leave the cab sit in so that it will covered in the degreaser. I just poor it in straight.
As pointed out below USE GLOVES!
This is a link to what I purchased last http://www.menards.com/main/housewares/cleaners/all-purpose/zep-commercial-ultrapurple-degreaser/p-1977297-c-7097.htm.
Only 10.00 a gallon and it will last for years most of the time, just strain the big flakes of paint out and reuse .